linux-kernel - Re: [PATCH 10/25] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

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Message-ID: <99d75a89-ef07-683a-761d-f800c53cc910@suse.cz>
Date:   Tue, 15 Jan 2019 14:18:03 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/25] mm, compaction: Ignore the fragmentation avoidance
 boost for isolation and compaction

On 1/4/19 1:49 PM, Mel Gorman wrote:
> When pageblocks get fragmented, watermarks are artifically boosted to
> reclaim pages to avoid further fragmentation events. However, compaction
> is often either fragmentation-neutral or moving movable pages away from
> unmovable/reclaimable pages. As the true watermarks are preserved, allow
> compaction to ignore the boost factor.
> 
> The expected impact is very slight as the main benefit is that compaction
> is slightly more likely to succeed when the system has been fragmented
> very recently. On both 1-socket and 2-socket machines for THP-intensive
> allocation during fragmentation the success rate was increased by less
> than 1% which is marginal. However, detailed tracing indicated that
> failure of migration due to a premature ENOMEM triggered by watermark
> checks were eliminated.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 57ba9d1da519..05c9a81d54ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2958,7 +2958,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
>  		 * watermark, because we already know our high-order page
>  		 * exists.
>  		 */
> -		watermark = min_wmark_pages(zone) + (1UL << order);
> +		watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
>  		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
>  			return 0;
>  
>